MR device with synthetic free layer structure

ABSTRACT

A magneto-resistive device having a large output signal as well as a high signal-to-noise ratio is described along with a process for forming it. This improved performance was accomplished by expanding the free layer into a multilayer laminate comprising at least three ferromagnetic layers separated from one another by antiparallel coupling layers. The ferromagnetic layer closest to the transition layer must include CoFeB while the furthermost layer is required to have low Hc as well as a low and negative lambda value. One possibility for the central ferromagnetic layer is NiFe but this is not mandatory.

FIELD OF THE INVENTION

The invention relates to the general field of magneto resistance (MR)with particular reference to the micro-structure of the free layer.

BACKGROUND OF THE INVENTION

Magnetic tunneling junction (MTJ) is a key component of both magneticrecording heads and magnetic random access memory (MRAM). A typical MTJstructure for a recording head or for an MRAM application isschematically illustrated in FIG. 1, as follows:

Buffer layer 11, antiferromagnetic layer (AFM) 12, an outer pinnedlayer, Ru, (neither shown) inner pinned layer 13 (the reference layer),tunnel barrier layer 14, free layer 15, and capping layer 16. In thisstructure, free layer 15 serves as the sensing layer which responds toexternal fields (specifically those stored in the media) while innerpinned layer 13 remains fixed and serves as reference layer. Theelectrical resistance through barrier (i.e. insulating) layer 14 varieswith the relative orientation of the free layer moment relative to thereference layer moment thereby converting magnetic information stored inthe media into electrical signals.

For effective operation as part of a magnetic recoding head, the basicrequirements for TMR sensors are as following:

1) Low resistance area product (RA)

2) High magneto resistance (MR) ratio

3) A soft free layer having low magnetostriction

4) Low interlayer coupling through the barrier layer.

5) Strongly pinned reference layer.

MgO-based MTJs are promising candidates for achieving high recordingdensity and/or high frequency application because their TMR ratio issignificantly higher than those of AlOx or TiOx based MTJs. S. Yuasa etal [1] and S. S. Parkin et al. [2] demonstrated that MR ratios around200% can be achieved at room temperature in epitaxialFe(001)/MgO(001)/Fe(001) and with polycrystallineFeCo/MgO/(Fe₇₀Co₃₀)₈₀B₂₀ MTJs.

Yuasa et al. [3] have also reported that TMR ratios as high as 410% atroom temperature can be achieved in fully epitaxialFe(001)/Co(001)/MgO(001)/Co structures. Meanwhile, D. D. Djayaprawira etal [4] showed that MTJs of CoFeB/MgO(001)/CoFeB structure made byconventional sputtering can also have a very high MR ratio (230%) withthe added advantage of greater feasibility and uniformity. For low RAapplications, the MR ratio of CoFeB/Mg/MgO/CoFeB MTJs can reach 138% atRA=2.4 ohm.μm² was achieved by K. Tsunekawa et al [5] by inserting aDC-sputtered metallic Mg layer in between the bottom CoFeB and therf-sputtered MgO, an idea initially proposed by T. Linn et al. [6] toprevent oxidation of the bottom electrode (CoFe) in a CoFe/MgO/reactivesputtering/NiFe structure. Also, Ta getter-presputtering prior to therf-sputtered MgO layer can achieve 55% TMR with an RA of 0.4 ohm.micron²as recently reported by Y. Nagamine et al. [7]. An alternative way toform a low RA MgO barrier is to deposit two metallic Mg layers with anatural oxidation process in between as we previously proposed for thebenefit of better process control and MRR uniformity. CoFeB material hasbeen used in MgO based MTJs to achieve a magnetically soft free layerhaving a high MR ratio. High MR ratio and low RA has been demonstratedin MgO MTJs with CoFeB free layer. It was also demonstrated thatinsertion of a thin CoFe layer between MgO barrier and CoFeB facilitatesgetting a high MR ratio even at low annealing temperatures (ca. 300°C.). However, there remains a concern that a CoFeB free layer will havea high positive magnetostriction coefficient (lambda).

There are several possible ways to reduce lambda in a CoFeB based freelayer. As shown elsewhere, lambda can be reduced by replacing CoFeB withCoB or by adjusting the CoFeB composition. However, magnetic softnessdeteriorated at the high annealing temperature needed to achieve a highMR ratio. To tackle this issue, a special annealing procedure wasdeveloped whereby a relatively high dR/R could be achieved while thefree layer was still soft. An alternative approach for reducing lambdais to add a NiFe layer, which has negative lambda and is magneticallysoft, on top of the CoFeB in the free layer. However, CoFeB/NiFe-typefree layer structure is not usable because direct contact between CoFeBand NiFe causes a drastic drop in MR ratio. H. Wang et al have proposedto use CoFe/CoFeB/Ta/NiFe as a free layer with high dR/R. In thisstructure, high dR/R can be achieved because the CoFeB is separated fromthe NiFe by a Ta insertion layer. The CoFe\CoFeB and NiFe layers aremagnetically coupled through orange-peel type coupling, which tend toalign magnetic moments to be parallel. However, this coupling isrelatively weak and, in the case of real devices, has to compete withmagnetostatic coupling from the edge of two layers which tend to alignthese two layers anti-parallel. As a result, magnetic noise for thiskind of structure is relatively high. So although signal amplitude ishigh, improvement in signal-to-noise ratio is limited.

REFERENCES

-   -   1. S. Yuasa et al “Giant room-temperature magnetoresistance in        single-crystal Fe/MgO/Fe magnetic tunnel junctions”, Nature        Materials 3, 868-871 (2004).    -   2. S. S. P. Parkin et al, “Giant tunnelling magnetoresistance at        room temperature with MgO (100) tunnel barriers”, Nature        Materials 3, 862-867 (2004).    -   3. S. Yuasa et al. “Giant tunneling magnetoresistance up to 410%        at room temperature in fully epitaxial Co/MgO/Co magnetic tunnel        junctions with bcc Co(001) electrodes”, Applied Physics Letters        89, 042505 (2006).    -   4. D. D. Djayaprawira et al. “230% room-temperature        magnetoresistance in CoFeB/MgO/CoFeB magnetic junctions”,        Applied Physics Letters 86, 092502 (2005).    -   5. K. Tsunekawa et al. “Giant tunneling magnetoresistance effect        in low-resistance CoFeB/MgO(001)/CoFeB magnetic tunnel junctions        for read-head applications” Applied Physics Letters 87, 072503        (2005).    -   6. T. Linn et al., “Method of forming a barrier layer of a        tunneling magnetoresistvie sensor”, U.S. Pat. No. 0,101,978 A1        (May 27, 2004).    -   7. Y. Nagamine et al. “Ultralow resistance-area product of 0.4        ohm**μm̂2 and high magnetoresistance above 50% in CoFeB/MgO/CoFeB        magnetic junctions” Applied Physics Letter 89 162507 (2006).    -   8. T. Zhao et al. Headway Invention Proposal HT05-045—“Low        Resistance Tunneling Magnetoresistive Sensor With        Natural-Oxidized Double MgO Barrier”    -   9. H. Wang et al Headway Invention Proposal HT07-040 “TMR Sensor        with Low magnetostriction Free layer    -   10. H. Wang et al Headway Invention Proposal HT07-028 “Novel        Free Layer Design for TMR/CPP Device”.        A routine search of the prior art was performed with the        following references of interest being found:

In U.S. Patent Application 2008/0117553, Carey et al. disclose a freelayer comprising CoFe/spacer/NiFe/spacer/CoFe where the spacer is Cu,Au, or Ag and where a third element can be alloyed with the CoFe orNiFe. Additional discussion of this reference appears later inconnection with TABLE I.

In U.S. Pat. No. 7,130,166, Gill shows a three-layer free layer wherethe first and third layers are CoFe/NiFe and the second layer is NiFe,but any of these layers can be NiFe, CoFe, or an CoFe/NiFe stack. Thelayers are separated by Ru. U.S. Pat. No. 7,141,314 (Zhang etal—Headway) teaches free layer laminates of CoFe and FeCo with spacerlayers of Cu. In U.S. Pat. No. 7,149,105, Brown et al. disclose a freelayer comprising NiFe and CoFeB separated by a nonmagnetic spacer suchas Ru, having a thickness of 2-30 Angstroms.

U.S. Patent Application 2007/0097561 (Miyauchi et al.) shows a freelayer comprising alloys of Co, Fe, Ni having a nonmagnetic layer inbetween while in U.S. Patent Application 2006/0291108, Sbiaa et al.describe a free layer containing a nonmagnetic spacer such as Ru, Rh,Ag, Cu.

SUMMARY OF THE INVENTION

It has been an object of at least one embodiment of the presentinvention to provide a magneto-resistive device having a large outputsignal as well as a high signal-to-noise ratio.

Another object of at least one embodiment of the present invention hasbeen that said device be any of CIP-GMR, CPP-GMR, MTJ, and TMR devices.

Still another object of at least one embodiment of the present inventionhas been to provide a process for forming said magneto-resistive device.

A further object of at least one embodiment of the present invention hasbeen for said process be fully compatible with existing processes forforming magneto-resistive devices.

These objects have been achieved by expanding the free layer into amultilayer laminate comprising at least three ferromagnetic layersseparated from one another by antiparallel coupling layers, whereby thecentral ferromagnetic layer has a magnetization direction that isantiparallel to that of the two outer layers.

Additionally, the first of these three ferromagnetic layers, the oneclosest to the transition layer, must include (or consist entirely of)CoFeB while the third layer is required to have low Hc as well as a lowand negative lambda value. One possibility for the central ferromagneticlayer is NiFe but this is not mandatory.

Processes for forming both the expanded free layer as well as the fulldevice are also described.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-section of a TMR stack of the prior art,including simple free layer 15.

FIG. 2 shows how free layer 15 of the prior art has been expanded into alaminate of three antiparallel coupled ferromagnetic layers.

FIG. 3 shows how the first of the ferromagnetic layers of FIG. 2 hasbeen further expanded into a three layer laminate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention discloses a free layer structure comprising atleast three magnetic layers whose magnetic moments are antiparallelcoupled through Ru layers. This is illustrated in FIG. 2 which shows howthe simple free layer 15 seen in FIG. 1 has been expanded to become amore complex structure. Seen in FIG. 2 are ferromagnetic layers mag1,mag2, and mag3 which have been designated as layers 21, 23, and 25respectively. The magnetizations of layers 21 and 23 are antiparallel toeach other as are those of layers 25 and 23, the required antiparallelcoupling between the two pairs of layers being provided by Ru layers 22and 24, respectively. Note that, while Ru is generally preferred,acceptable antiparallel coupling can also be achieved by using othermaterials such as Rh, Cu, Cr, or Ir in place of Ru. As with Ru, theirthickness should be in a range of from 4-10 Å.

Mag1 can be made of single layer or combination of: CoxFe1-x(x=0˜100%),(CoxFe1-x)1-yBy (x=0˜100%, y=10˜40%) or their alloys with third or forthelements, such as Ni, Ta,Mn, Ti,W, Zr, Hf, Tb. For example, CoFeB,CoFe\CoFeB or CoFe\CoFeB\CoFe. The thickness for each layer may be 1˜40A. Materials or structures that give higher dR/R are preferred.

Mag2 can be a single layer or a combination of: NixFe1-x (x=0˜100)CoxFe1-x(x=0˜100%) and their alloys with third or fourth elements, suchas Ni, Ta,Mn, Ti,W, Zr, Hf, Tb or B. For example, CoFe, NiFe orCoFe\NiFe\CoFe. The thickness for each layer may be 1˜30 A. Materials orstructures with low or negative lambda that promote strong couplingthrough spacers are preferred.

Mag3 can be a single layer or combination of: NixFe1-x (x=0˜100)CoxFe1-x(x=0˜100%), their alloys with third or fourth elements, such asNi, Ta,Mn, Ti,W, Zr, Hf, Tb or B. For example, NiFe, CoFe\NiFe orCoFe\NiFe\CoFe. The thickness for each layer may be 1˜80 A. Materials orstructures with negative lambda and low Hc are preferred.

The TMR barrier can be MgO, MgZnO, ZnO, Al₂O₃, TiOx, AlTiO, HfOx, ZrOxor any combination of these.

As illustrated in FIG. 3, Mag1 may be sandwiched between two layers, 31and 33, of CoFe, each about 3 Å thick. Layer 31 enhances the MR ratiofor anneal temperatures less than about 300° C. while layer 33 is forstrong AP coupling through layer 22.

The characteristics required for the five main layers are summarized inTABLE I below. For comparison purposes we have also included the mainfeatures of the structures disclosed in U.S. 2008/011753, mentionedearlier in connection with our prior art search:

TABLE I 2008/0117553 PRESENT INVENTION COMMENTS MAG1 CoFe(40-60%),CoFeAl CoFeB, CoFe/CoFeB or Must include CoFeB or CoFeSiCoFe/CoFeB/CoFe, or alloy with 4^(th) element MAG2 NiFe (2-25%) CoFe,NiFe, or Not limited to NiFe CoFe/NiFe/CoFe MAG3 CoFe(40-60%), CoFeAlNiFe, CoFe/NiFe or Low Hc and negative or CoFeSi CoFe/NiFe/CoFe lambdarequired* APC LAYERS Cu, Au, Ag; 1-5 Å Ru, Rh, Cu, Cr, Ir; Must promoteAPC, 4-10 Å ruling out Au & Ag *Cannot be accomplished by CoFe (40-60%)single layer

In TABLE II below, we show data for a number of different TMR stackshaving different free layer structures. Samples A and B, with freelayers of CoFe3\CoFeB\Ru and CoFe3\CoFeB\CoFe\Ru respectively, have highMR ratios (up to 67% at RA 1.5) but cannot be used because of their veryhigh positive lambda values (˜1.4E-5).

Inserting Ru\NiFex\Ru\NiFey on top allows us to significantly reducemagnetostriction while still maintaining a high MR ratio and goodmagnetic softness (see samples C, D, E, and F). The amount ofmagnetostriction and the magnetic moment can be readily tuned byadjusting the thickness of the two NiFe layers. Sample F has near zeromagnetostriction, a Bs slightly higher than the reference (sample B),and an Hc of only 5.6 Oe.

In samples D and F, a thin CoFe layer has been inserted for the purposeof enhancing the coupling strength between CoFeB and NiFe layers. Forexample, the coupling field for CoFe3\CoFeB20\Ru8\NiFe35 is 800 Oe andwhich increases to 1700 Oe for CoFe3\CoFeB20\CoFe3\Ru8\NiFe35. Couplingstrength could be even further improved by inserting another thin CoFelayer under the NiFe or by replacing NiFe with CoFe,

Sample G is an example of a synthetic free layer with thicker top NiFelayer and a thinner middle NiFe layer, demonstrating that Bs can beadjusted independently of total thickness and lambda.

TABLE II Sample Bs Hc ID Free Layer Structure (nW) (Oe) Lambda ACF3\CFB20\Ru 0.34 4.9 1.4E−05 B CF3\CFB20\CF3\Ru 0.37 4.5 1.4E−05 CCF3\CFB20\Ru\NF15\Ru\NF20 0.40 5.3 3.8E−06 DCF3\CFB20\CF3\Ru\NF15\Ru\NF20 0.44 5.2 3.9E−06 ECF3\CFB20\Ru\NF35\Ru\FN40 0.41 5.8 −2.3E−07 FCF3\CFB20\CF3\Ru\NF35\Ru\NF40 0.44 5.6 2.0E−08 GCF3\CFB20\CF3\Ru\NF20\Ru\NF50 0.59 4.2 −4.2E−07MgO MTJ with CoFe\CoFeB and CoFe\CoFeB free layers.The structure of the full TMR stack wasTa\Ru\IrMn\CoFe\Ru\CoFe\MgO\FL\capping.The films were annealed at 280° C. for 5 hours in a magnetic field of 8KOe.

Note that the free layer of the present invention may also be used inother similar MR structures (such as CIP-GMR, CPP-GMR and TMR),particularly those that need high lambda material to boost dR/R, whilestill retaining low free layer softness.

1. A process to form a free layer as part of a magneto-resistive device,comprising: depositing on a barrier layer a first layer of ferromagneticmaterial (MAG1); depositing on MAG1 a first antiparallel coupling layer;depositing on said first antiparallel coupling layer a second layer offerromagnetic material (MAG2); depositing on MAG2 a secondantiparallel-coupling layer; depositing on said second antiparallelcoupling layer a third layer of ferromagnetic material (MAG3) having acoercivity less than about 5 Oe and a negative lambda value that is morenegative than about −2×10⁻⁶; and magnetizing all three ferromagneticlayers in a manner such that MAG1 and MAG3 become magnetized parallel toone another and antiparallel to MAG2.
 2. The process recited in claim 1wherein said first and second antiparallel coupling layers are Ru. 3.The process recited in claim 1 wherein said first and secondantiparallel coupling layers are selected from the group consisting ofRh, Cu, Cr, and Ir.
 4. The process recited in claim 1 wherein the stepof depositing MAG1 further comprises: depositing on said substrate afirst layer of CoFe to a thickness in the range of from2to 10 Å;depositing on said first layer of CoFe a layer of CoFeB to a thicknessin the range of from 10 to 40 Å; and depositing on said layer of CoFeB asecond layer of CoFe to a thickness in the range of from 2 to 10 Å. 5.The process recited in claim 1 wherein MAG2 is selected from the groupconsisting of CoFe, NiFe, and a CoFe/NiFe/CoFe laminate.
 6. The processrecited in claim 1 wherein MAG3 is selected from the group consisting ofCoFe, NiFe, and a CoFe/NiFe/CoFe laminate.
 7. The process recited inclaim 1 wherein the step of magnetizing all three ferromagnetic layersin a manner such that MAG1 and MAG3 become magnetized parallel to oneanother and antiparallel to MAG2, is achieved without the need for aspecial magnetizing process.
 8. A free layer that is part of amagneto-resistive device, comprising: a first layer of ferromagneticmaterial (MAG1); on MAG1, a first antiparallel coupling layer; on saidfirst antiparallel coupling layer, a second layer of ferromagneticmaterial (MAG2); on MAG2, a second antiparallel coupling layer; on saidsecond antiparallel coupling layer, a third layer of ferromagneticmaterial (MAG3) having a coercivity less than about 5 Oe, and a negativelambda value that is more negative than about −2×10⁻⁶; and said threeferromagnetic layers being magnetized in a manner such that MAG1 andMAG3 are magnetized parallel to one another and antiparallel to MAG2. 9.The free layer described in claim 8 wherein said first and secondantiparallel coupling layers are Ru.
 10. The free layer described inclaim 8 wherein said first and second antiparallel coupling layers areselected from the group consisting of Rh, Cu, Cr, and Ir.
 11. The freelayer described in claim 8 wherein MAG1 further comprises: a first layerof CoFe having a thickness in the range of from 2 to 10 Å; on said firstlayer of CoFe, a layer of CoFeB having a thickness in the range of from10 to 40 Å; and on said layer of CoFeB a second layer of CoFe having athickness in the range of from 2 to 10 Å.
 12. The free layer describedin claim 8 wherein MAG2 is selected from the group consisting of CoFe,NiFe, and a CoFe/NiFe/CoFe laminate.
 13. The free layer described inclaim 8 wherein MAG3 is selected from the group consisting of CoFe,NiFe, and a CoFe/NiFe/CoFe laminate.
 14. A magneto-resistive device,comprising: an antiferromagnetic layer on a substrate: a magneticallypinned reference layer on said antiferromagnetic layer; a transitionlayer on said magnetically pinned reference layer; a first layer offerromagnetic material (MAG1) on said transition layer; on MAG1, a firstantiparallel coupling layer; on said first antiparallel coupling layer,a second layer of ferromagnetic material (MAG2); on MAG2, a secondantiparallel coupling layer; on said second antiparallel coupling layer,a third layer of ferromagnetic material (MAG3) having a coercivity lessthan about 5 Oe and a negative lambda value that is less than about2×10⁻⁶; a capping layer on MAG3; and said three ferromagnetic layersbeing magnetized in a manner such that MAG1 and MAG3 are magnetizedparallel to one another and antiparallel to MAG2.
 15. Themagneto-resistive device described in claim 14 wherein said transitionlayer is copper whereby said magneto-resistive device is selected fromthe group consisting of CIP-GMR and CPP GMR devices.
 16. Themagneto-resistive device described in claim 15 wherein said first andsecond antiparallel coupling layers are selected from the groupconsisting of Ru, Rh, Cu, Cr, and Ir.
 17. The magneto-resistive devicedescribed in claim 15 wherein MAG1 further comprises: a first layer ofCoFe having a thickness in the range of from 2 to 10 Å; on said firstlayer of CoFe, a layer of CoFeB having a thickness in the range of from10 to 40 Å; and on said layer of CoFeB a second layer of CoFe having athickness in the range of from 2to 10 Å.
 18. The magneto-resistivedevice described in claim 15 wherein MAG2 and MAG3 are each selectedfrom the group consisting of CoFe, NiFe, and a CoFe/NiFe/CoFe laminate.19. The magneto-resistive device described in claim 14 wherein saidtransition layer is a tunneling barrier layer selected from the groupconsisting of MgO, MgZnO, ZnO, Al₂O₃, TiOx, AlTiO, HfOx, ZrOx, includingany combination of these group members, whereby said magneto-resistivedevice is selected from the group consisting of MTJ and TMR devices. 20.The magneto-resistive device described in claim 19 wherein said firstand second antiparallel coupling layers are selected from the groupconsisting of Ru, Rh, Cu, Cr, and Ir.
 21. The magneto-resistive devicedescribed in claim 19 wherein MAG1 further comprises: a first layer ofCoFe having a thickness in the range of from 2 to 10 Å; on said firstlayer of CoFe, a layer of CoFeB having a thickness in the range of from10 to 40 Å; and on said layer of CoFeB a second layer of CoFe having athickness in the range of from 2 to 10 Å.
 22. The magneto-resistivedevice described in claim 19 wherein MAG2 and MAG3 are each selectedfrom the group consisting of CoFe, NiFe, and a CoFe/NiFe/CoFe laminate.